The present invention relates to a method for making a piezoelectric thin film resonator using hydrogen implant layer splitting and wafer bonding to transfer thin, high quality piezoelectric material layers to an optimized substrate.
To date, the primary approach to making thin film resonators has been to deposit thin layers of AlN or ZnO using techniques such as sputtering. The piezoelectric properties of deposited polycrystalline thin films of ZnO or AlN are generally not as good as those for single-crystal bulk substrates such as ZnO, AlN, quartz, LiNbO3, or LiTaO3, or for high temperature sintered ceramic materials such as lead zirconium titanate (PZT) or lead lanthanum zirconium titanate (PLZT). Typically, the Q of films made using the deposited thin film materials is smaller than one would expect for thin layers of bulk single-crystal piezoelectric material. Additionally, the thickness of the resonator material must be well controlled, since the frequency of resonance is greatly dependent upon the thickness of the piezoelectric material. A piezoelectric material develops an electric polarization when mechanically stressed by stress. In the converse effect, an applied electric field produces a mechanical distortion (strain) on a piezoelectric material.
Both single-crystal bulk substrates and high temperature sintered ceramic piezoelectric materials typically have improved piezoelectric properties, compared to thin film grown piezoelectric materials. Typical single-crystal piezoelectric substrates include lithium niobate, quartz, lithium tantalate, zinc oxide, tellurium oxide, lead zirconium titanate, lead lanthanum zirconium titanate, and relaxor ferroelectrics.
There are typically two types of thin film resonators. In one approach, a thin layer of piezoelectric material is sandwiched between two metal electrodes on a substrate in selected locations that contains an acoustic Bragg reflector mirror consisting of quarter wavelength layers of high acoustic impedance and low acoustic impedance materials. This type of thin-film resonator is solidly mounted. In another type of thin film resonator, a thin layer of piezoelectric material is sandwiched between two metal electrodes on a substrate that has an air cavity below the resonator.
Bruel, in U.S. Pat. No. 5,374,564, describes a method for making thin semiconductor material films by hydrogen implantation and heating to cause the semiconductor to split at the location of the peak of the hydrogen ion implant. It has been found experimentally that there are a number of techniques to either reduce the required hydrogen ion implantation dose or to reduce the temperature needed to cause hydrogen ion implantation substrate layer splitting process to work. One technique involves the use of a high-pressure nitrogen gas steam directed towards the side of a silicon substrate into which a high dose hydrogen ion implantation has been made. It has been experimentally found that the splitting process can occur in single-crystal semiconductor materials at room temperature for the case of a silicon substrate into which a high hydrogen ion implantation dose has been made using the high pressure nitrogen gas stream that is directed toward the edge of the wafer method. It has also been found experimentally that a helium ion implantation made in combination with a hydrogen ion implantation can be used to achieve a lower total implanted dose for the substrate layer splitting process to occur for a given anneal temperature. It has also been found experimentally that a lower substrate layer splitting temperature is achieved for the case that a hydrogen ion implantation is made into a silicon substrate having a high boron concentration. The high boron concentration can be incorporated into a silicon substrate by ion implantation. The lower temperature for hydrogen ion implantation substrate layer splitting to occur is obtained both for the case that the boron implant is annealed and for the case that the boron implant is unannealed.
Thin films of piezoelectric material are sometimes grown on single crystal substrates such as magnesium oxide or strontium titanate. The frequency of operation of the piezoelectric resonator depends strongly on the thickness of the piezoelectric material. Piezoelectric material thicknesses less than 2 microns is typically required for resonant frequency greater than 1 GHz. To date there has not been a reliable and inexpensive method for producing high frequency resonators from single-crystals of piezoelectric material.
It is an object of the present invention to produce a thin film single-crystal piezoelectric material using hydrogen implant layer splitting techniques.
It is an object of the present invention to provide a method for producing a piezoelectric resonator.
It is another object of the present invention to transfer thin layers of piezoelectric material to an appropriate substrate using hydrogen ion implant layer splitting approach and wafer bonding.
It is yet another object of the present invention to transfer thin layers of piezoelectric material to a substrate that contains CMOS or GaAs circuitry.
According to the present invention, a thin layer of single-crystal piezoelectric material can be produced by implanting hydrogen into a single-crystal piezoelectric material, wafer bonding to a substrate, and heating the material to a temperature sufficient to cause hydrogen gas in the piezoelectric material to expand and to split the crystal at the location of the peak of the hydrogen implant. Alternately, a thin layer of single-crystal piezoelectric material can be produced by implanting hydrogen into a single-crystal material, wafer bonding and directing a high pressure gas stream at the side of the wafer to cause splitting at the location of the peak of the hydrogen ion implantation.
In another embodiment of the present invention, thin layers of high quality single-crystal piezoelectric material, high temperature sintered piezoelectric material, or high quality thin film grown material are transferred to an appropriate substrate using hydrogen ion implant layer splitting and bonding. The substrate to which the thin piezoelectric material layer is transferred may contain CMOS or GaAs circuitry.
When the substrate contains CMOS or GaAs circuitry, the circuitry on the surface of the GaAs or CMOS substrate may be covered with an oxide. The oxide is then planarized using chemical mechanical polishing, and the thin film resonator material, thin film resonator having metal electrode, or thin film resonator having metal electrode and acoustic Bragg reflector materials is transferred to the GaAs or CMOS circuit using wafer bonding and hydrogen ion layer splitting.